Coastal Contractor

Soundings

Engineered Wood in Coastal Environments

Q:

I build custom homes in a harsh
coastal environment, and I'm concerned about the durability of
laminated veneer lumber (LVL), parallel-strand lumber (PSL), and
glue-laminated beams in these houses. Often the buildings sit empty
through a long winter season, and I'm worried that a leak that
isn't immediately detected might damage big interior load-bearing
beams and columns. Also, I'm curious to know what experience tells
about treated-wood engineered beams for coastal conditions, whether
in outdoor exposure or indoor situations.

A:

Architect and builder Andrew P.
DiGiammo responds: I used to have the same concerns you do, but
after many years of using these materials in waterfront houses in
Massachusetts and Rhode Island, I've laid those worries to
rest.

After 15 years in service, treated Parallam posts supporting a
three-story beach house deck are as good as new (left). Treated
Parallam girders supporting the main house's first-floor system
likewise show no ill effects from weather or insects
(right).

When I first used these products 15 years ago, it was on a custom
home in the dunes, with a three-story outdoor deck facing the
Atlantic Ocean. I designed the building with a concrete pier
foundation, using pressure-treated Parallams for first-floor
girders and a floor frame of wood I-joists (both manufactured by
Trus Joist Macmillan, now a part of Weyerhaeuser).

I had two concerns back then. My main worry was that some of the
Parallam girders were going to be exposed forever under the floor,
where they might be attacked by salt-laden moisture in the air (the
kind of salt air that I have seen tear up the lockset on an entry
door within a year). But I was also concerned about the exposure of
all the other framing material in that house while it was under
construction and before it was enclosed.

I also planned to support the entire three-story outdoor deck with
pressure-treated Parallam posts. Those would be facing the weather
directly, and I didn't know how they would hold up.

Now, 15 years later, I know the answer. All the Parallam beams
under the house are still in fine shape, as are the Parallam posts
holding up the outdoor deck — they look as good as they did
the day we installed them. So I'm fully confident in treated
Parallam members in any harsh coastal exposure. In fact, I have
gained enough confidence that for the last house

I built on elevated open piers, I specified Parallam beams without
the pressure treatment. Knowing that they'll be sheltered under the
house, not exposed to bulk water, I trust that they'll hold up
well.

As for the wood I-joists, none of the material I used in that house
suffered damage from being exposed to weather during construction
— nor has any of the wood I-joist material I've used in
houses since then. As it happens, just the other day I went back to
that house in the dunes to replace some roofing, and I noticed some
pieces of the wood I-joists I used for framing, stacked under the
house. (The owner had asked me to set aside some of that material
for some reason or other, but he never used it.) After 15 seasons,
those exposed and untreated I-joist scraps are now showing their
age: the wood has turned gray and has rotted in places. The glued
joints, however, have not yet let go. This conforms with all my
other observations of this type of product — the material
decays as a piece of lumber normally would, depending on the wood
species it is made of, before it falls apart as an engineered piece
of lumber.

My experience doesn't extend to pressure-treated LVL or
glue-laminated timbers. I have, however, seen untreated LVL beams
suffer from severe moisture conditions. My company recently rebuilt
an old mill building in Fall River, Mass., that was built over a
dirt crawlspace. Five years before we got there, someone had gone
underneath and supported the floor with LVL girders. In that moist
environment, those beams completely decayed and delaminated —
just as badly as a piece of spruce would have. The lesson is that
if it's not pressure treated, an LVL has no more moisture
resistance than a piece of softwood lumber. So for severe moisture
conditions, you want something treated.

In situations that call for moisture or weather resistance, I
always reach for treated Parallams — and I can tell you that
on my jobs, treated Parallams have been able to handle coastal
moisture conditions just fine.

Andrew P. DiGiammo is an architect and custom
builder based in Assonet, Mass.

Engineered Wood and Preservative
Treatments

To augment the lessons of design-builder Andrew DiGiammo's
experience, Coastal Contractor called up Peter Laks, a professor in
the School of Forestry and Wood Products at Michigan Technological
University. Laks is an expert on wood preservative treatments and
on the durability of wood-based composite building materials.

According to Laks, the decay resistance of an untreated wood
composite member — whether it's a parallel-strand material
like Parallam, a laminated veneer beam, or a glue-laminated timber
— depends primarily on the wood species. Softwoods in general
hold up better over time than hardwoods, Laks says (especially
among woods used for engineered framing lumber). "For instance,
some Parallams are made from yellow poplar," he explains. "If it's
not treated, that will be less durable than a Parallam made from
southern yellow pine or Douglas fir, in terms of fungal decay
resistance and termite resistance."

For outdoor uses, any composite member should be pressure treated,
Laks emphasizes. But he adds that different engineered materials
have differing abilities to absorb treating chemicals. Parallam,
for instance, is made from strands of wood and has "a pretty open
structure," he says. "There is a lot of void space within the
composite. It's more porous. That means it will absorb water much
faster than a solid piece of wood of equivalent dimensions, and
that also means that you can pressure-treat it more easily. You'll
get really good penetration of preservative, because of that
permeability, in a Parallam — better than an equivalent-size
piece of solid wood."

LVL material, which is made up of thin veneers, is different. "All
those glue lines can be quite impermeable," says Laks. "Generally,
you get a better treated product from Parallam than from
LVL."

Glue-lams are made from 2-by dimensional stock — 2x4 or 2x6
softwood. Sometimes the boards are treated before they are glued up
into a beam, but more typically the beam is made first and then
treated. Either way, says Laks, the member will take treatment
pretty much the way regular softwood lumber would, because there
are fewer glue lines to block the penetration of the chemical than
in an LVL. By the same token, a glue-lam made of wood that takes
treatment readily, such as southern pine, will be easier to treat
successfully than a glue-lam made with a relatively impermeable
wood such as Doug fir.

Manufacturing and treating defects can occasionally happen, so it's
important to verify that good procedures were followed. When it
comes to wood treating, says Laks, there are industry standards:
the American Wood Preservers Association publishes guidelines for
proper treating of composite members, with specific requirements
for parallel-strand lumber, laminated veneer lumber, glue-laminated
timbers, and wood structural panels. Some companies, however, forgo
the AWPA process in favor of obtaining an Evaluation Report (ER)
from the International Code Council Evaluation Service
(ICC-ES).

End users can verify lumber characteristics from markings applied
to the lumber. Typically, treated wood members bear two stamps: one
that pertains to the original "white wood" lumber, giving the name
of the company that manufactured the lumber (often with a mill
number), along with the wood species and grade and the logo of the
inspection agency, and a second stamp or stapled-on tag indicating
the date of treatment, the type of chemical applied, the level of
chemical retained by the wood, the treating standard followed
(either an AWPA standard number or an ER number), and the intended
use of the product (ground contact, above ground, and so on).
— Ted Cushman